UVI cells were isolated by C. Waldren as a UV sensitive CHO mutant. The cells have an abnormality in post-replication repair - DNA synthesis on a damaged template. Since post-replication repair is dependent on recombinational repair in bacteria, we decided to determine if these cells have an abnormality in recombinational repair. We have recently shown in mammalian cells that a small fraction of pyrimidine dimers are exchanged into DNA synthesized after UV by a recombination process somewhat analogous to recombination repair in bacteria. The fraction of dimers exchanged in UVI cells was similar to that of the wild type cells.